1. Field of the Invention
The present invention relates to a zoom lens and an imaging apparatus including the zoom lens, and more particularly to a small-sized zoom lens having a high magnification change ratio, and an imaging apparatus including the zoom lens.
2. Description of the Related Art
Imaging apparatuses including a digital still camera using solid state imaging sensors have become widespread. In recent years, small-sized imaging apparatus systems using small-sized solid state imaging sensors including Micro Four Thirds have been rapidly developing. Accordingly, requirement on a miniature zoom lens having high imaging performance have increased depending on market needs for a zoom lens adjusting a focal length depending on an object as an optical systems in the imaging systems. Moreover, in recent years, market needs for a telephoto zoom lens with a high magnification change ratio like a focal length exceeding 300 mm in terms of 35-mm film have been increased.
In an interchangeable lens for a small-sized imaging system of such type, focusing by a contrast method instead of a conventional phase difference method has been employed. In the phase difference method, a focus position is determined on the basis of distance information estimated by a phase difference sensor. In contrast, a peak position of contrast of a subject image determined on an imaging sensor surface is detected while a focus lens group is moved along an optical axis to detect the peak position as a focus position in the contrast method. As described above, in the contrast method, moving of a focus lens group in order to detect a peak position of contrast is necessary. As a result, a focus speed tends to reduce as compared with the phase difference method. Accordingly, to achieve high speed autofocusing in the contrast method, high speed movement of a focus lens group is necessary.
A zoom lens disclosed in Japanese Patent Laid-Open No. 2009-265652 achieves high imaging performance all over a zooming area by increasing the number of movable lens groups in zooming to increase degrees of freedom of aberration correction. At the same time, a rear focus method is applied to reduce a size of a focus lens group in a radial direction to achieve weight reduction of the focus lens group, and high speed autofocusing is achieved.